The present invention relates to security for programmable non-volatile memories (PNVM) and, in particular, to such a memory and associated circuitry equipped to handle and act upon, in a secure manner, a security status data stored within the memory itself.
Programmable non-volatile memories (e.g. electrically erasable programmable read-only memory and flash memory circuits) are well-known. In some applications, such as in control of safety functions in automobiles, security circuitry for read/write protection of the contents of PNVM circuits is commonly used. In this way, end-consumers are prevented from altering the contents of these memories and causing potentially life-threatening conditions. For example, security circuitry would commonly be used to protect the contents of an PNVM containing a program to control a vehicle""s anti-lock brake system.
Conventionally, such security circuitry includes non-volatile security register circuitry, separate from the PNVM memory array itself, and the contents of which determine the security status of the PNVM contents. A major disadvantage of using non-volatile register circuitry for security functions is that it uses a large amount of chip area, and also that it requires special complex circuitry apart from the memory circuitry itself.
A memory system in accordance with the invention includes a non-volatile memory to contain a security indication and program instructions. A volatile register is also provided.
Memory interface logic circuitry operates responsive to the contents of the volatile register, to selectively allow access to the memory. Security indication copying circuitry receives a reset signal for the memory system. In response to the reset signal, the copying circuitry causes the security indication to be copied into the volatile register as the contents thereof.